This invention relates to storage batteries, particularly to methods of determining the state of charge of such batteries.
It is necessary or at least highly advantageous in many applications to determine the current state of charge (SOC) of a storage battery. At first, efforts in this regard concentrated on measuring and evaluating the changes in electrolyte density, either directly by mechanical and optical methods, or indirectly via the open-circuit voltage of the battery, or the individual cells, coupled to the acid density.
With advancing progress in the field of electronic data acquisition and processing, methods of charge balancing using electricity meters have come to the fore. These methods exhibit very good success in the case of batteries that are subjected to complete charging and discharging with only rare intermediate chargings. Errors can arise from inaccuracies in measurement and computation, parasitic charge losses in the storage battery, the aging of the battery and extractable battery capacities changing with the level of the charging or discharging current. However, it has not heretofore been possible with the aid of such integrating methods to make a reliable statement on the current performance readiness (the product of current times voltage) of the battery, since, for example, the change in the internal resistance is a function of the complete or partial cycles preceding with regard to time interval and current level.
A number of the weak points mentioned in the charge balancing method can be partially overcome by forming mathematical models of the battery. Methods using mathematical models (descriptive equations or mathematically representable equivalent circuit diagrams), which are evaluated on electronic computers with stipulation of a set of battery-typical basic data or equivalent circuit diagram components, make it possible to detect the state of charge of the battery operating simultaneously in parallel with the model. A limiting factor of these methods is not only the quality of the basic model, but the quality of the input parameters with regard to their correspondence to the battery under consideration.
Balancing the output voltage calculated via the model with the actual battery voltage under the conditions of identical temperature and identical current throughput has proved to be particularly useful as an evaluation method. This type of evaluation also permits estimation of the aging processes in the storage battery.
Representative disadvantages of methods based on mathematical models include an enormous rise in model complexity, particularly when the aim is to calculate the effects of short-term changes in charge and when large quantities of charge are also picked up by or discharged from the batteries over relatively long time intervals.
The problem is in that the most varied charging or discharging conditions are possible for the starter battery, depending on the motor vehicle cycles, distributed statistically with regard to duration and speed profile, on which there are likewise superimposed statistically distributed power consumption profiles of the electric consumers installed in the automobile, and must be detected by the state of charge determining unit.
It would, therefore, be highly advantageous to provide a method for determining the state of charge of a storage battery which detects large exchanges of charge in relation to the battery capacity, rest periods and standby phases, as well as battery aging.
The invention is direct to a method for determining the state of charge of a storage battery comprising simultaneously applying at least two methods having different steps to determining state of charge of the battery; weighting individually obtained results of the different methods based on their respective reliability in the respective current or former operating situation of the storage battery; and determining a weighted mean value of the individual methods thus obtained as a final output variable.